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    <meta charset="UTF-8">
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    <title>Document</title>
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    <script>
        var qufan=true
        console.log(!qufan);
        //以下六个值取反后为true
        console.log(!false);
        console.log(!null);
        console.log(!undefined);
        console.log(!NaN);
        console.log(!0);
        console.log(!"");
        //且运算（&&）(都要满足)
        console.log(10>5&&10>20);//false
        //或运算||（只需满足任意一项）
        console.log(10>5||10<5);//true
    </script>
</body>
</html>